Ozone is a very powerful oxidizing agent for organic substances and for inorganic compounds which contain elements with several oxidation levels. Beside multiple applications in the field of chemistry, it has been introduced decades ago in the field of water treatment. The high costs of investment and operation, however, restrict the application possibilities.
Even though there are, theoretically, many possibilities for producing ozone, only the production of ozone by silent electrical discharges has gained any significance. This is true for large stationary ozone producing installations as well as for small transportable ones. Ozone producing devices which operate according to this principle consist essentially of two electrodes which are separated from one another by a dielectric and a gas chamber. A high voltage alternating current is applied to the electrodes. A the same time, one allows oxygen or an oxygen containing gas to flow through the gas chamber. A high voltage electrical discharge hereby occurs between the electrodes without the formation of sparks and without bright light displays. This high voltage discharge leads to ozone formation. Two types of ozone producing devices which use this principle are known, namely, plate type ozone producing devices according to which the electrodes consist of parallel plates, and tubular ozone producing devices according to which the electrodes consist of concentric tubes. The invention is concerned with the latter type of ozone producing devices.
Numerous constructions of tubular ozone producing devices are known; German patents DE-PS No. 17 67 109 and DE-PS No. 32 21 895 show examples of typical constructions. Many efforts have been directed toward improving the efficiency of ozone producing devices, especially by increasing the intensity of the field. In order to accomplish this purpose, for example, the surface of the electrodes has been roughened with a coating as disclosed in DE-PS No. 12 40 831 or the dielectric was equipped with electrically conductive islands as disclosed in EU-PS No. 00 19 307. Special constructions have also become known. For example, EU-PS No. 00 18 318 discloses an inner electrode which consists of wire wound around a cylindrical core while the outer electrode is designed, in the usual manner, as a tube. DE-PS No. 156,531, on the other hand, discloses a spiral wound outer electrode and an inner electrode which is designed as a tube or as a rod.
Spiral shaped electrodes require, because of their construction, relatively large discharge chamber/spaces and, for this reason, yield low and irregular discharge densities as well as low ozone concentrations. Furthermore, in the case of spiral shaped outer electrodes, cooling with liquid is not possible when the gap between the outer electrode and the dielectric is to be used as a discharge chamber.
With the ozone producing device in usage today, high voltages discharge electrodes are the predominant type being used. These have either a metallized dielectric made from, for example, glass, ceramic or plastic, or else from metal plates or metal tubes coated with the dielectric. The high voltage is thus applied to the metallic layer. A directly or indirectly cooled electrode, which can, in certain cases, also carry a dielectric coating, is usually opposite the high voltage discharge electrode. The heat resulting from the gaseous discharge is supposed to be channeled away via the cooling medium. With a good cooling system, the yield of ozone can be increased. For this reason, attempts are made in many cases, to cool the high voltage electrode by means of a second, expensive cooling circuit.
With the prevailing constructions, the heat occurring as a result of the silent electrical discharge can only be carried to the cooled outer electrode by diffusion and convection. Because of the relatively low field strength during ozone production, the gas discharge path can only be impacted at relatively high gas volumes and at very low velocities. At these low gas velocities, the amount of heat transferred is practically equal to zero so that no heat can be carried off via the flowing gas. Unlike, as with plate ozone producing device, with tubular ozone producing devices, one is faced with great difficulty in assuring the maintenance of an exact distance. This is primarily caused by fluctuations in the diameter of the tube as well as by bends. It is thus not possible, even with the use of centering elements, to keep the tube's distance constant over the entire length.
Because of the rigidity of the tubes, which have diameters of 30 mm or more, an elastic reshaping by means of the centering elements is also not possible. As a result of these inaccuracies, highly varied discharge densities occur which, in turn, lead to considerable reductions of the ozone yield.